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It is reported that the signatory works for a leading medical research organisation in Cambridge.

Charlie is suffering from a rare form of mitochondrial disease, RRM2B deficiency, which causes a wide range of disabilities.

Chris Gard and Connie Yates, Charlie's parents

The letter says the therapy is still at an experimental stage and that more tests are needed, but says "...there is insufficient time to perform these studies to justify treatment for Charlie Gard."

It adds that "reconsideration of treatment for Charlie Gard is respectfully advocated."

Great Ormond Street Hospital has now applied for a fresh hearing in the case, and this is scheduled to take place in the High Court on Monday.

Connie Yates (PA)

Charlie's parents Connie Yates and Chris Gard, both in their 30s and from West London, want to take him to a hospital in America, but lost a lengthy legal battle after judges ruled in favour of doctors at Great Ormond Street, who argued the treatment would not improve the 11-month-old's quality of life.

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Leaving the hospital last night, Charlie's mother told Sky News: "We're hopeful and confident that Charlie may get a chance now."

The disease affects cells responsible for energy production and respiration and leaving him unable to move or breathe without a ventilator. The therapy is not a cure.

Protesters supporting the family's plea for treatment

Great Ormond Street describes experimental nucleoside therapies as "unjustified" but its decision comes after two international hospitals and their researchers contacted them "as late as the last 24 hours" to say they have "fresh evidence about their proposed experimental treatment".

Under a High Court ruling, GOSH is forbidden from allowing Charlie to be transferred for nucleoside therapy anywhere.